1. Field of the Invention
The invention relates to a method for manufacturing carbon bodies by heating blanks containing pyrolizable substances in a chamber oven, in which volatile pyrolysis products are formed and decomposed, and thermal energy for heating the blanks is generated by combustion of fluid fuels inside the chamber.
2. Description of the Prior Art
In manufacturing carbon bodies it is known to mix comminuted coke, graphite, carbon black and other substances consisting substantially of carbon with one or several binders from the group coal tar pitch, petroleum pitch, tar, synthetic resin and, to a small extent, also carbohydrates. The resultant mixture is formed into bodies by extrusion, screw extrusion die pressing or on vibration machines. In first approximation, the blanks consist of a closed binder matrix, in which solid carbon grains are dispersed. By heating the blanks, the binders are subjected to pyrolysis, whereby a binder coke skeleton joining the primary carbon grains together and volatile products, mainly hydrocarbons, are formed from the binder. The carbon bodies obtained are porous, primarily because of the pyrolysis losses, and it is known, according to another method, to impregnate the porous carbon bodies with one of the substances used as the binder. It is then necessary to subject the bodies to a second heat treatment and to coking the impregnation medium.
The heat treatment is performed for less sensitive blanks primarily in annular chamber kilns which consist of a large number of chambers connected in tandem. Controlling this compound oven is difficult, since parameter changes in one chamber of necessity affect the adjacent chambers. Predetermined operating conditions are easier to maintain in individual chamber ovens, called chamber ovens in the following, which are accordingly used for firing blanks which are sensitive to firing and also when firing with a high heating-up rate. A chamber oven of this type is described, for instance, in U.S. Pat. No. 3,048,382. The blanks to be fired are arranged in packing material consisting substantially of coke powder, and the thermal energy required for heating-up the chamber contents is generated by combustion of a fluid fuel in the space formed by the surface of the packing material and the lid of the oven. The flue gas flows through vent holes arranged in the packing material and is returned at least in part, as are the volatile pyrolysis products. The pyrolysis products are, especially at high heating-up rates, not at thermal equilibrium, so that secondary cracking products such as retort coke and particularly soot are formed at the hotter oven surfaces of the vent holes, oven walls etc., which narrow down the free flow cross section more and more. In principle, the deposition or precipitation of carbon-containing substances can be prevented by combustion of the volatile pyrolysis products, for which purpose, oxygen must be fed to the circulated flue gas. Any increase of the oxygen content, however, in the actual concentration range means increased explosion danger. For instance, at an oven temperature of 500.degree. C. a typical composition of the circulated flue gas is as follows (in volume percent): H.sub.2, 2.1%; CO, 0.8%; CH.sub.4, 0.8%; polyaromatics, 0.7%; O.sub.2, 1.6%; CO.sub.2, 9.0%; N.sub.2, 67.7%; H.sub.2 O, 16.5%. The explosion limit is in general not reached for the individual component, but may be reached for the mixture of the flammable components. Under the conditions mentioned as an example, the lower explosion limit is about 4.7% by volume.
The same difficulties exist regardless of the form and type of the chamber oven. Particularly obstructive are the encrustations for the operation of an oven type called shuttle kiln (bogey hearth oven), in which the material to be fired is stacked on a carriage outside the oven chamber and is run into the chamber.